(1) Field of the Invention
The present invention relates to sealed storage batteries such as Nickel Cadmium (Ni—Cd) storage batteries, and manufacturing methods therefor.
(2) Description of the Related Art
Sealed batteries are widely used as driving power supply sources in power tools, electric bicycles, electric motorbikes, and so on. Representative of sealed batteries are alkaline secondary batteries such as Nickel Cadmium (Ni—Cd) batteries and Nickel Metal Hydride (Ni-MH) storage batteries, nonaqueous secondary batteries such as lithium ion batteries, and primary batteries. When put to such use, a battery is required at times to have good charge and discharge properties under conditions such as large current, and therefore it is necessary to reduce the internal resistance in the battery as much as possible and provide a stable current path.
However, a typical manufacturing method for a sealed battery (see Japanese Laid-Open Patent Applications 2002-93455 and H10-79243) is performed in the following manner in the case of a cylindrical battery. First, a power generating element (electrode assembly) is made by winding a positive-electrode plate and a negative-electrode plate together with a separator therebetween. Then power collecting bodies, each having a circular main body, are positioned on the electrode of each polarity, facing each other so as to cover the top and bottom ends of the electrode assembly, and this arrangement is packaged in a tube-shaped metal outer can. The positive-electrode current collector body is connected to a seal cover that is at an opening end terminal (positive terminal) of the outer can, and the negative-electrode current collector body is connected to the bottom (negative terminal) of the outer can. Next, after injecting an electrolyte into the metal outer can, resistance welding electrodes are applied to the battery from the top and bottom directions, and an instantaneous, large current is applied, thereby simultaneously resistance welding the positive-electrode plate of the electrode assembly and a rectangular tab extending from the positive-electrode current collector body with the terminal provided on the seal cover, and the negative-electrode plate of the electrode assembly with the outer can. Note that this resistance welding method is referred to as a “direct welding method” hereinafter. The seal cover is then fitted so as cover a opening in the outer can, and subject to caulking or the like, so as to seal the outer can, thereby completing the sealed battery.
Japanese Laid-Open Patent Application 2002-231216 discloses a technique for improving the shape of the positive-electrode current collector body in order to perform the welding effectively in a state where the seal cover is placed over the opening of the outer can, and to reduce the resistance value of the current path by shortening the length of the tab.
Here, in the outer can, an insulating member called an insulating ring, as shown in the elevation view in FIG. 6, is arranged on top of the electrode assembly so that the positive-electrode current collector body fits within the area of the ring. The insulating ring is provided, among other purposes, to position the electrode assembly inside the outer can, to prevent shorts between the outer can and the positive-electrode current collector body, and to prevent the electrolyte leaking.
However, the following problem occurs in a conventional battery with the described structure.
The tab of a conventional positive-electrode current collector body is fitted inside the insulating ring by being bent back toward the main body of the current collector body. This can result in the tip of the bent tab being in direct contact with the main body.
However, the existence of this part where the tab and the main body are in contact means that there is a risk that the load current value of the resistance welding will be inconsistent (so-called “leakage current” occurs) when using the aforementioned direct-welding method in manufacturing, and, consequently, that the resistance welding will not be able to be performed reliably.
If resistance welding is not carried out satisfactorily every time, the welding strength will be different for each battery. This leads to inconsistency in performance between batteries.
For this reason, prevention of unnecessary contact between the tab and the main body of the positive-electrode power conducting body as a result of resistance welding remains an issue.
Note that the same kind of problem also potentially occurs when the terminal of the seal cover is negative and the seal cover is connected to the negative-electrode current collector body.